The second form of false motion is the better known problem of lifter "pump-up." The hydraulic lifter's plunger is continually under hydraulic pressure from the engine's oiling system. Under demanding circumstances such as high rpm, the valvetrain can partially unload. This unloading can occur during the onset of valve float, during spring surge, or with valve bounce on closing. Unloading can also occur when the effective spring load on the valvetrain is drastically diminished while the cam's lobe rotates over the nose at high rpm. The hydraulic lifter's plunger will quickly pump up any time the force of the oil acting on the hydraulic piston exceeds the force of the valvetrain on the lifter's plunger. This will cause the lifter to be temporarily overextended into a condition referred to as lifter "pump-up." The overextended lifter causes the valve to be held slightly off the seat when the camshaft is on its base circle, effectively hanging up the valves.

The aftermarket has developed some variations on the standard-issue hydraulic lifters. One of the first refinements was the introduction of anti-pump-up lifters. The concept is as simple as it is effective. In an anti-pump-up lifter, the light-duty retaining clip at the end of the hydraulic lifter's internal plunger travel is replaced with a heavier, more positive stop.

When used in conjunction with an adjustable valvetrain, an anti-pump-up lifter can be set so the internal plunger is at or near the top of its range of travel when the camshaft is on its base circle. When running, the anti-pump-up lifter is essentially adjusted so the piston is already pumped all the way up against the stop, eliminating the possibility of the plunger overextending. An adjustable valvetrain is, of course, required to utilize an anti-pump-up lifter as intended. Anti-pump-up lifters may also include changes to the lifter's valving or clearances to alter the bleed-down characteristics, although current theory holds that "stiffer" is better.

At what point can instability with a hydraulic lifter begin to hinder performance? The answer, unfortunately, is combination specific. Valvetrain weight and geometry, pushrod deflection, preload adjustment, spring load, and the cam profile's smoothness and intensity are some of the factors, along with rpm, that can upset a hydraulic lifter's ability to maintain valve control. Even oil viscosity and temperature have been reported to make a difference.

Though there are too many variables to pinpoint the rpm capability of a hydraulic-lifter camshaft, extensive experience in the use of hydraulic cams can suggest basic guidelines. Depending upon the camshaft/valvetrain/spring combination, standard hydraulic lifters can operate effectively in the 5,500-6,000-rpm range. Typically, anti-pump-up lifters can raise the rpm potential by 500-1,000 rpm more. Certainly, some have far exceeded these numbers, while other combinations experience problems at even more conservative levels.

Solid Solution
Solid lifters are as the name implies-solid. There is no internal mechanism to take up clearance, and in fact, they require clearance to operate properly. This clearance is called the valve lash. Why, you may ask, is lash required? As the cam comes around to the base circle, the lifter must unload the valvetrain and allow the valve to close. Theoretically, this occurs at zero lash, but some additional clearance is needed to give the solid-lifter valvetrain a little wiggle room to compensate for dimensional changes due to heat expansion.

The real beauty of a solid-lifter setup is in its simplicity. Essentially, it's a machined chunk of steel with no moving parts. There's nothing to foul up the valvetrain operation. Set up correctly, a solid is about as reliable as a brick, because it is about as complicated as one. Sometimes simplicity is a tough attribute to beat.